In radio frequency (RF) receivers, for example in low frequency (LF) receivers used e.g. for short range devices such as remote controls, or security systems, such as vehicle security systems, e.g. tyre pressure monitoring systems (TPMS), demodulation and baseband processing for extracting a digital data signal from a received input signal is usually done using analog circuits. The receiver comprises a detector circuit which has a baseband filter for providing a demodulated signal and a data slicer used to transform the demodulated analog signal to a digital one. This principle is illustrated in FIG. 1: An input signal is filtered using a baseband filter 12. A demodulated input signal is then applied to a data slicer 16, for example a comparator, and an average filter 18, that provides an average signal 20 to the data slicer. The data slicer compares the voltage of the signal 14 with a reference value which is for example the average value of the average signal 20 provided by the average filter.
Defining the averaging time constant of the average signal may be a compromise between speed and sensitivity, because fast averaging leads to high ripple in the average signal which reduces sensitivity, while slow averaging leads to low ripple thus improving sensitivity but at the expense of a longer delay until a valid extracted digital data signal 22 can be provided.
FIG. 2 shows a schematic diagram of a digital data signal extracted from a baseband input signal using a prior art approach of average filter pre-charging. The average filter is pre-charged once 32, e.g. at the level present at the input of the filter. An analog input signal 24 comprising noise 26 and an average signal 28 are compared for extracting a digital data signal 30. The average signal 28 slowly approximates a final average value of the input signal 24. Therefore, the data contained in the digital data signal may be corrupted due to the noise, and the duty cycle of the digital data signal 30 may not be correct due signal shaping of the input signal 24.